when to use a URN for xmlns

Question

For a "hello world" type xml document:

<?xml version="1.0" encoding="UTF-8"?>
<!-- see http://www.w3.org/TR/REC-xml-names/ -->
<bk:book xmlns:bk="urn:loc.gov:books" xmlns:isbn="urn:ISBN:0-395-36341-6">
  <bk:title>Cheaper by the Dozen</bk:title>
  <isbn:number>1568491379</isbn:number>
</bk:book>

you can arbitrarily define urn's on the fly for custom purposes?

Answer 1

I found this answer quite useful, and it led me to this article which seems to say that yes you can make them up as you like, and indeed you SHOULD, if you aren't going to place a document online for a given URL.

Answer 2

In short, yes. You can make 'em up as you like.